In silicon-chip implementations including a local oscillator and a power amplifier, the injection of a periodic signal may lead to a locking or pulling phenomena. The electronic circuits, such as RF transceivers or a device for frequency synthesis techniques, suffer from frequency pulling when a pulsed modulation signal is applied. During transmission of the pulses, for the RF transceiver having a transmitter and a receiver, having the power amplifiers and local oscillators in the same substrate affects the transmitted signal by generating an oscillation frequency change between the receiver local oscillator and the transmitted signal frequency.
The effect of the undesirable frequency deviation in the transmitted signal results in a loss of correlation that increases error in the demodulated signal from the receiver. A switching element in an electronic circuit having a local oscillator may generate similar effects by causing pulling effects.
In particular, the present disclosure relates to a silicon integrated circuit. Applicable fields may include integrated circuits on semiconductors, e.g., CMOS, Bipolar, BiCMOS technologies, radio frequency and microware applications, circuits including oscillators, power amplifiers and/or receivers, and integrated circuits for radar applications using pulse modulation techniques, for example.
A transceiver in accordance with the prior art is shown in FIG. 1, which includes a transmitter Tx and a receiver Rx. The transmitter Tx includes a local oscillator LO that generates a radio frequency carrier f1 which is modulated by a first switch S1 and amplified by a power amplifier PA. The transmitted signal is a train of pulses modulated using frequency, amplitude, pulse time duration, pulse position or other modulation techniques. The signal received from the receiver Rx is amplified by a low-noise amplifier LNA and down-converted to the intermediate frequency IF by an interposed adder, and by using a second switch S2.
The receiver Rx may also be switched to blind or turned off when the transmitter Tx generates the train of modulated pulses. The switches S1 and S2 are driven by a pilot signal PT and its complementary pilot signal PTn, respectively, as shown in FIG. 2.
Some prior art approaches provide a local oscillator LO by controlling the tuning voltage VTUNE through an open loop approach or PLL systems. These approaches, which are convenient in certain circumstances, also suffer from certain drawbacks. In particular, the compensation of the pulling frequency is reduced in high frequency oscillator circuits when the frequency is approximately of 24 GHz and/or 77-79 GHz, as in radar systems, i.e., having fast pulses. The local oscillator LO is subjected to the power coupling and/or impedance loading of the power amplifier PA that generates leakage current through the substrate or through circuit pathways of the chip.
This leakage current affects the oscillation frequency of the local oscillator LO wider than the ideal frequency during the open (ON) and closed (OFF) phases of the switch modulator S1. This pulling effect leads to a source of error for the transmitted signal which is found in a frequency deviation Δfer=f1−f2, from f1 frequency of carrier signal to f2 during the pulses transmission close phase of the switch modulator, as shown in FIG. 2. Moreover, it should be noted that even a control system such as a phase lock loop PLL cannot be used to compensate the frequency deviation for high-frequency applications wherein fast pulses are provided.
The document “A study of injection locking and pulling in oscillators” by Razavi B. IEEE Journal of Solid-State Circuits, vol. 39, 2004, pages 1415-1424, relates to an RF transceiver TX where the output signal of the power amplifier PA contains large spectral components in the vicinity of ωLO, leaking through the package and the substrate to the local oscillator LO and causing frequency deviation. The leakage of the power amplifier PA may be considered as a sinusoidal injection current that modifies the frequency where the local oscillation LO occurs.
It may be particularly desirable to improve the architecture of the local oscillator LO to reduce the pulling effect in an efficiency mode during high frequency modulation. It may be desirable to obtain a transceiver having frequency coherence between the transmitted/received signals and the carrier signal in a relatively simple implementation.
Moreover, a versatile approach that may be used in oscillator circuits integrated on semiconductors, e.g., CMOS, Bipolar, BiCMOS, radio frequency and microwave applications and integrated circuits for radar applications using pulse modulation techniques, may be desirable. Furthermore, it also may be desirable to provide an approach to increase the performance of current radar systems where high power pulses are transmitted that are used, for example, in automotive and industrial applications.